ECAT — Engineering College Admission Test (UET)

ECAT Chemistry — Stoichiometry and Atomic Structure (Set 1)

Take the ECAT Chemistry — Stoichiometry and Atomic Structure (Set 1): 30 multiple-choice questions (40 minutes) with instant scoring and a full answer…

What this test covers

30 ECAT Chemistry

Sample questions with answers

The first 5 questions of this test, with the correct answer and explanation.

Q1. How many hydrogen atoms are present in 0.25 mol of CH₄?
- A. 6.02 × 10²³ ✓
- B. 1.505 × 10²³
- C. 2.408 × 10²⁴
- D. 3.01 × 10²³
Answer: A. 6.02 × 10²³
Explanation: Each CH₄ has 4 H atoms: 0.25 × 4 = 1 mol of H atoms = 6.02 × 10²³.
Q2. What is the mass of 3.01 × 10²² molecules of NH₃?
- A. 0.17 g
- B. 0.85 g ✓
- C. 8.5 g
- D. 1.7 g
Answer: B. 0.85 g
Explanation: 3.01 × 10²² / 6.02 × 10²³ = 0.05 mol; 0.05 × 17 g/mol = 0.85 g.
Q3. The percentage of nitrogen by mass in urea, CO(NH₂)₂, is closest to:
- A. 26.7%
- B. 23.3%
- C. 46.7% ✓
- D. 53.3%
Answer: C. 46.7%
Explanation: Molar mass of urea = 60 g/mol and it contains 28 g N, so %N = 28/60 × 100 = 46.7%.
Q4. What volume does 8 g of O₂ gas occupy at STP?
- A. 5.6 dm³ ✓
- B. 2.8 dm³
- C. 11.2 dm³
- D. 22.4 dm³
Answer: A. 5.6 dm³
Explanation: 8 g / 32 g mol⁻¹ = 0.25 mol; 0.25 × 22.4 dm³ = 5.6 dm³.
Q5. 4 g of H₂ is reacted with 16 g of O₂ according to 2H₂ + O₂ → 2H₂O. What mass of water is formed?
- A. 9 g
- B. 36 g
- C. 18 g ✓
- D. 20 g
Answer: C. 18 g
Explanation: H₂ = 2 mol, O₂ = 0.5 mol. O₂ needs only 1 mol H₂, so O₂ is limiting and gives 1 mol H₂O = 18 g.

The other 25 questions

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Q6. Heating 50 g of CaCO₃ (CaCO₃ → CaO + CO₂) actually produced 19.8 g of CO₂. What is the percentage yield?
- A. 85%
- B. 90%
- C. 95%
- D. 80%
Q7. A compound contains 40% C, 6.67% H and 53.33% O by mass, and its molar mass is 90 g/mol. Its molecular formula is:
- A. C₃H₆O₃
- B. C₂H₄O₂
- C. CH₂O
- D. C₆H₁₂O₆
Q8. Which sample contains the largest number of atoms?
- A. 1 g of H₂
- B. 16 g of O₂
- C. 4 g of He
- D. 18 g of H₂O
Q9. Boron has two isotopes, ¹⁰B (20%) and ¹¹B (80%). Its relative atomic mass is:
- A. 10.8
- B. 10.2
- C. 11.0
- D. 10.5
Q10. In the mass spectrum of an element, the relative abundance of each isotope is shown by:
- A. the position of its peak on the m/e axis
- B. the accelerating voltage applied
- C. the strength of the magnetic field used
- D. the height of its peak (ion current)
Q11. How many electrons are present in the ion ⁵⁶Fe³⁺ (Z = 26)?
- A. 30
- B. 26
- C. 29
- D. 23
Q12. Using Bohr's model, the radius of the third orbit (n = 3) of the hydrogen atom is about:
- A. 0.53 Å
- B. 4.76 Å
- C. 1.59 Å
- D. 2.12 Å
Q13. The energy of the electron in the second orbit (n = 2) of hydrogen is (E₁ = −1312 kJ mol⁻¹):
- A. −146 kJ mol⁻¹
- B. +328 kJ mol⁻¹
- C. −656 kJ mol⁻¹
- D. −328 kJ mol⁻¹
Q14. The wavelength of the line emitted when an electron in hydrogen falls from n = 3 to n = 2 is about (R = 1.097 × 10⁷ m⁻¹):
- A. 122 nm
- B. 434 nm
- C. 486 nm
- D. 656 nm
Q15. Electrons in a sample of hydrogen atoms fall from n = 5 to the ground state by all possible routes. The maximum number of different spectral lines is:
- A. 4
- B. 5
- C. 10
- D. 15
Q16. Which series of the hydrogen spectrum lies in the ultraviolet region?
- A. Brackett series
- B. Balmer series
- C. Paschen series
- D. Lyman series
Q17. The de Broglie wavelength of an electron (m = 9.11 × 10⁻³¹ kg) moving at 7.27 × 10⁵ m s⁻¹ is about (h = 6.626 × 10⁻³⁴ J s):
- A. 1.0 nm
- B. 0.1 nm
- C. 10 nm
- D. 100 nm
Q18. What is the maximum number of electrons in a subshell with azimuthal quantum number l = 3?
- A. 18
- B. 10
- C. 14
- D. 6
Q19. Which set of quantum numbers (n, l, m, s) is NOT allowed?
- A. 3, 2, +2, −½
- B. 2, 1, −1, +½
- C. 2, 2, 0, +½
- D. 1, 0, 0, −½
Q20. The ground-state electronic configuration of chromium (Z = 24) is:
- A. [Ar] 3d⁴ 4s²
- B. [Ar] 3d⁵ 4s¹
- C. [Ar] 3d⁶
- D. [Ar] 4s² 4p⁴
Q21. How many unpaired electrons does a ground-state sulphur atom (Z = 16) have?
- A. 4
- B. 2
- C. 6
- D. 0
Q22. According to the (n + l) rule, which of these orbitals is filled first: 4p, 5s, 4d or 3d?
- A. 3d
- B. 4d
- C. 5s
- D. 4p
Q23. In the nitrogen atom, the three 2p electrons occupy three different p orbitals with parallel spins. This follows from:
- A. the Aufbau principle
- B. Pauli's exclusion principle
- C. Heisenberg's uncertainty principle
- D. Hund's rule
Q24. Pauli's exclusion principle states that in an atom:
- A. no two electrons can have the same set of four quantum numbers
- B. electrons fill orbitals in order of increasing energy
- C. position and momentum cannot both be known exactly
- D. electrons pair only after degenerate orbitals are half-filled
Q25. The energy of a photon of wavelength 400 nm is about (h = 6.626 × 10⁻³⁴ J s, c = 3 × 10⁸ m s⁻¹):
- A. 2.48 × 10⁻¹⁹ J
- B. 4.97 × 10⁻¹⁹ J
- C. 4.97 × 10⁻²⁸ J
- D. 9.94 × 10⁻¹⁹ J
Q26. Moseley's work on X-ray spectra showed that the square root of the frequency of the X-rays emitted by an element is directly proportional to its:
- A. number of neutrons
- B. valency
- C. atomic number
- D. atomic mass
Q27. In a discharge tube, the e/m value of the positive (canal) rays is greatest when the tube contains:
- A. helium gas
- B. oxygen gas
- C. neon gas
- D. hydrogen gas
Q28. What mass of NaOH is needed to neutralize 0.1 mol of H₂SO₄ completely?
- A. 2 g
- B. 4 g
- C. 8 g
- D. 16 g
Q29. What mass of iron can be obtained from 160 g of Fe₂O₃ (Fe = 56, O = 16)?
- A. 56 g
- B. 112 g
- C. 80 g
- D. 120 g
Q30. How many electrons are present in 1.8 g of water?
- A. 6.02 × 10²²
- B. 1.8 × 10²⁴
- C. 6.02 × 10²³
- D. 1.08 × 10²³